Cortical mechanisms for the segregation and representation of acoustic textures.

Wellcome Trust Centre for Neuroimaging, Institute of Neurology, University College London, London WC1N 3AR, United Kingdom. t.overath@nyu.edu

Abstract

Auditory object analysis requires two fundamental perceptual processes: the definition of the boundaries between objects, and the abstraction and maintenance of an object's characteristic features. Although it is intuitive to assume that the detection of the discontinuities at an object's boundaries precedes the subsequent precise representation of the object, the specific underlying cortical mechanisms for segregating and representing auditory objects within the auditory scene are unknown. We investigated the cortical bases of these two processes for one type of auditory object, an "acoustic texture," composed of multiple frequency-modulated ramps. In these stimuli, we independently manipulated the statistical rules governing (1) the frequency-time space within individual textures (comprising ramps with a given spectrotemporal coherence) and (2) the boundaries between textures (adjacent textures with different spectrotemporal coherences). Using functional magnetic resonance imaging, we show mechanisms defining boundaries between textures with different coherences in primary and association auditory cortices, whereas texture coherence is represented only in association cortex. Furthermore, participants' superior detection of boundaries across which texture coherence increased (as opposed to decreased) was reflected in a greater neural response in auditory association cortex at these boundaries. The results suggest a hierarchical mechanism for processing acoustic textures that is relevant to auditory object analysis: boundaries between objects are first detected as a change in statistical rules over frequency-time space, before a representation that corresponds to the characteristics of the perceived object is formed.

Auditory stimulus. Example of a block of sound with four spectrotemporal coherence segments showing absolute coherence values for each segment and the corresponding change in coherence between the segments.

Areas showing an increased haemodynamic response as a function of increasing absolute coherence (blue) and increasing change in coherence (red). Results are rendered on coronal (y = −24, top) and tilted (pitch = −0.5, middle (superior temporal plane) and bottom (STS)) sections of participants' normalised average structural scans. The bar charts show the mean contrast estimates (± SEM) in a sphere with 10 mm radius around the local maximum corresponding to the six levels of absolute coherence (blue) and the six levels of change in coherence (red). Change in coherence levels are pooled across ‘positive’ and ‘negative’ changes so as to show the main effect of change in coherence magnitude. The charts nearest the brain show the mean response in the sphere around the local maxima for increasing change in coherence; those at the sides show the mean response in the sphere around local maxima for increasing absolute coherence. Note that the placement of the identifying letter in the brain sections only approximate the precise stereotactic [x y z] coordinates at the bottom corner of each chart, since no single planar section can contain all the local maxima simultaneously.

Increasing vs. decreasing coherence change between texture segments. Coronal (y = −32) section showing areas that display a stronger increase for changes across which coherence increased than for changes across which coherence decreased. The bar charts at the sides show the mean contrast estimates (± SEM) in a sphere with 10 mm radius around the local maxima in left and right TPJ, respectively, for the different conditions.